1. Field of the Invention
The present invention relates to an image processing method that may be adopted in edge enhancement processing and noise removal processing executed on an image.
2. Description of Related Art
The technologies for enhancing edges through multi-resolution representation proposed in the related art include that disclosed in U.S. Pat. No. 6,754,398. Through the art disclosed in U.S. Pat. No. 6,754,398, an edge enhancement effect is achieved by subjecting the signals in LH, HL and HH high-frequency subbands obtained through multi-resolution transformation, e.g., wavelet transformation, to nonlinear gradation conversion with the input/output characteristics thereof expressed as a monotonously increasing function, such as that shown in FIG. 9 included in U.S. Pat. No. 6,754,398 so as to extract edge components from the signals, synthesize the LH, HL and HH edge components and add the synthesized edge components to the original image. In addition, the technology disclosed by the inventor of the present invention in International Publication No. 2007/114363 pamphlet represents an example of edge enhancement whereby the original image is projected into frequency spaces that include the LL low-frequency subbands in addition to the LH, HL and HH high-frequency subbands forming a complete system, thus allowing for more redundancy and assuring a higher level of freedom, edge detection is executed in the individual subbands and the edges in the original image are enhanced based upon the edge component generated by synthesizing the edge components detected in the various subbands.
The regular edge enhancement processing executed in the real space, such as unsharp mask processing, is known to be inherently prone to a problematic phenomenon called “ringing” induced by overshoot or undershoot, which occurs readily near an edge. This issue is addressed in, for instance, Japanese Laid Open Patent Publication No. 2005-353102 by first executing nonlinear conversion with the input/output characteristics thereof expressed as a monotonously increasing function on the high-frequency component data and then incorporating the high-frequency component data having undergone the nonlinear conversion into a smoothed image, as indicated in FIGS. 5 and 13 and expressions 23 and 23′ included in the publication. In addition, U.S. Pat. No. 5,666,443 discloses a technology whereby limiter processing is executed so as to ensure that the values of the image signals having undergone the edge enhancement fall within the range defined by the largest value and the smallest value of local image signals in the pre-edge enhancement state.
In another edge enhancement method of the related art, an edge component is extracted from the difference between an edge-enhanced image and the original image, an ultimate edge component is calculated by multiplying the extracted edge component by a weighting coefficient that will apply greater weight over an edge area and the edges in the original image are enhanced by adding the ultimate edge component to the original image, as disclosed in U.S. Pat. No. 6,628,842. In the edge enhancement method disclosed in U.S. Pat. No. 6,373,992, a component containing an edge component and a noise component in combination is extracted from the difference between an edge-enhanced image and a smoothed image, the edge component is extracted by multiplying the component having been extracted by a noise fluctuation inhibition coefficient corresponding to edge intensity having been detected elsewhere and the edges are enhanced by adding the extracted edge component into the smoothed image. Namely, these publications each disclose a technology whereby the weighting coefficient is set so as to apply greater weight on the edge component only around edges, in order to minimize the presence of the noise component in the edge component.
U.S. Pat. No. 6,754,398 discloses expression (21) as shown below, which represents a process for achieving edge enhancement and noise removal simultaneously. The expression indicates that the noise component contained in the edge component is reduced by executing nonlinear conversion on noise components extracted at various resolution levels prior to multi-resolution synthesis during the process of generating a noise component SN1 so as to apply a greater extent of attenuation on each component with significant noise amplitude and generate an synthesized noise component SNp1 constituted with components having only small noise amplitudes and eliminating the synthesized noise component SNp1 from the edge component.Sproc=Sorg+β(Sorg)*(SH1−SNp1)−α(Sorg)*SN1
The technologies in the related art that enable edge enhancement or edge enhancement combined with noise removal include a method whereby edge components having been extracted based upon multi-resolution representation are synthesized and the synthesized edge component is added into the original image as disclosed in U.S. Pat. No. 6,754,398 and International Publication No. 2007/114363 pamphlet (filed by the inventor of the present invention). In the technology disclosed in U.S. Pat. No. 6,754,398, the target data are directly processed in the gradation space in which the original image is expressed and the edge component to be added to the original image is first multiplied by a weighting coefficient corresponding to the brightness level of the original image. In the art disclosed in International Publication No. 2007/114363 pamphlet, more advanced noise component/edge component extraction performance is assured for purposes of noise removal and edge enhancement by converting the target data to data in a uniform color•uniform noise space assuming gradation characteristics different from those of the input image, executing noise removal or edge enhancement and then converting the data back to data in the initial color space.
Japanese Laid Open Patent Publication No. 2005-353102, on the other hand, discloses a dynamic range compression technology in a field other than edge enhancement or noise removal, i.e., compression of the dynamic range of x-ray images in radiology. In the technology disclosed in Japanese Laid Open Patent Publication No. 2005-353102, the high-frequency component in the original image is obtained, the high-frequency component is converted in conformance to the pixel values indicated in the original image and the slope of the gradation conversion curve and then the converted high-frequency component is incorporated in the original image. Namely, in order to improve the visibility of the effective information concerning the subject constituted of a low-frequency component buried in noise present in, particularly, a dark area, gradation density distribution width is adjusted with flexibility while inhibiting noise emphasis.
Another technology in the related art that enables noise removal and edge enhancement through multi-resolution representation is disclosed in U.S. Pat. No. 6,754,398. In the technology disclosed in U.S. Pat. No. 6,754,398, noise components and edge components are extracted from high-frequency subband images expressed in multi-resolution representation, e.g., the LH, HL and HH high-frequency subband images obtained through wavelet transformation, an synthesized noise component and an synthesized edge component are generated by synthesizing the extracted noise components and edge components through multi-resolution synthesis, noise removal is executed by subtracting the noise component from the original image and edge enhancement is achieved by adding the edge component to the original image. Since the individual noise components are extracted by projecting the target data into frequency spaces corresponding to the high-frequency subbands LH, HL and HH appearing to constitute a complete system, gaps may be formed between the frequency bands depending upon the noise removal filters used in conjunction with the various subbands. The presence of such gaps is bound to cause incomplete extraction of noise components.
This issue is addressed in International Publication No. 2007/116543 pamphlet filed by the inventor of the present invention, which discloses a method for preventing incomplete noise extraction by executing noise extraction in redundant frequency spaces with the data also projected into frequency spaces of low-frequency subbands LL that are generated sequentially and synthesizing the two types of noise components, one corresponding to the low-frequency subbands and the other corresponding to the high-frequency subbands. The publication discloses that the extent of loss of image structure attributable to the noise removal should be minimized by adjusting the noise removal intensity in correspondence to the characteristics of the frequency distribution in the luminance component plane and the characteristics of the frequency distribution in the chrominance component planes which are different from each other, so as to execute high-intensity noise removal for the high-frequency subbands and low-intensity noise removal for the low-frequency subband in the luminance component plane, so as to execute low-intensity noise removal for the high-frequency subbands and high-intensity noise removal for the low-frequency subband in the chrominance component planes.
International Publication No. 2007/114363 pamphlet filed by the inventor of the present invention discloses that the weighting coefficient k (0≦k≦1) used to adjust the noise removal intensity level for the low-frequency (L) subbands and the high-frequency (H) subbands may be set to k:1:1:1 for LL, LH HL and HH in the luminance component plane for purposes of noise component synthesis. The value assumed for k, via which the noise frequency characteristics can be altered freely, greatly affects the appearance of the image resulting from the noise removal. Since the preferred extent of noise removal varies from person to person, the art disclosed in the publication allows the user to select a value for this parameter through a user interface. In addition, as is the noise removal, the edge enhancement is executed by synthesizing edge components extracted both from the low-frequency subbands and the high-frequency subbands while adjusting the edge enhancement intensity for the low-frequency subbands and the edge enhancement intensity for the high-frequency subbands via a weighting coefficient that can be altered freely during the synthesis processing.